1. Field
Embodiments of the present invention may generally relate to a communication system based on a single carrier frequency division multiple access (SC-FDMA). More particularly, embodiments of the present invention may relate to devices and methods of cancelling interference by adopting a subtractive interference cancellation method and a multiuser multiple input multiple output (MU-MIMO) scheme.
2. Background
For various demands on multimedia services in a wireless communication environment, a lot of progress has been made in transmitting a large amount of data at high speeds. The wireless communication environment has lower reliability compared to a wired communication environment due to propagation attenuation, shadowing, time-varying noise, multipath Interference, multiuser Interference.
An antenna device adopting a multiple input multiple output (MIMO) scheme includes multiple antennas in a transceiver. Each antenna receives independent information so that it is possible to transmit data at high speed within limited frequencies. In the MIMO antenna device, receiving signals of one antenna may have interference caused by other antennas. To remove the interference in the received signals, two methods can be adopted. A first method is a kind of non-subtractive interference cancellation, known as a “one-shot multiple-antenna detection method.” In the first method an inverse matrix is calculated with correlations among channels of the multiple antennas to remove the interference. A second method is known as subtractive interference cancellation. According to the second method, signals transmitted from one transmitting antenna are detected. The detected signals are used to reconstruct received signals of an antenna of interest to remove the interference in the receiving signals.
In non-subtractive interference cancellation, rank or orthogonal factors of a channel matrix are important elements which determine the performance of the MIMO antenna device. For example, if the channel matrix has good orthogonality the receiving signals have less mutual interference signals and the MIMO antenna device shows good performance. However, in the case of bad orthogonality, lots of the mutual interference signals may be included in the receiving signals, and the performance of the MIMO antenna device is degraded. In order to prevent degradation of the MIMO device, an estimation of mutual orthogonal components is needed to perform scheduling when the non-subtractive interference cancellation is adopted.
The subtractive interference cancellation has been widely studied in a code division multiple access (CDMA) system. In this method, the signals transmitted from multiple terminals (user equipments) are analyzed and used like multiple transmitting antennas. Thus, multiple access interference can be reduced, and capacity and performance of the CDMA system can be improved. However, as the number of terminals increases, it is not easy to configure a real system due to complexity.
The 3rd Generation Partnership Project (3GPP) long term evolution (LTE), of which standards have been settled, adopts a downward orthogonal frequency division multiple access (downward OFDMA) scheme and an upward SC-FDMA scheme instead of the CDMA scheme in transmitting and receiving signals in consideration of suitability for wideband frequencies. Also, to maximize the efficiency at each frequency, a single input multiple output (SIMO) scheme and the MIMO scheme may be applied to the up-link and the down-link. Specifically, in the up-link, a multi-user MIMO (MU-MIMO) may be applied to enhance the frequency efficiency.
FIG. 1 shows comparative performances of two terminals randomly selected in the up-link, which adopts one of the MU-MIMO scheme and the SIMO scheme, respectively. Results shown in FIG. 1 are obtained in assumptions that coding rates of quadrature phase shift keying (QPSK) and of 16 quadrature amplitude modulation (QAM) are 0.67 and 0.76, respectively. 432 subcarriers are used in this example. Referring to FIG. 1, additional power of 4 dB (QPSK) and 7 dB (16 QAM) is needed when scheduling signals received from the terminal of the SIMO scheme in a scheme of the MU-MIMO.
FIG. 2 shows a schematic diagram of scheduling and resource allocation of a user equipment UE in an up-link. In allocating the resources, the SIMO scheme is applied to UE1 and UEs 4 to 6 and the MU-MIMO scheme is applied to UE 2 and UE3. In the case of UE2 and UE3 which are scheduled in accordance with the MU-MIMO scheme, it is not able to discriminate the UE2 and UE3 by demapping the subcarriers since UE2 and UE3 use a common frequency band. Thus, an equalizer should be used to discriminate UE2 and UE3. One consideration may be to adopt a scheme in which a scheduler checks mutual relationships among channels of users and groups the users into pairs. The users that have less of a mutual relationship are included in one pair. However, this scheme cannot be configured in a real system, since it is extremely rare that there is no mutual relationship between user channels due to frequency selective fading generated in wide band communication. Further, in the case of an LTM system, which has been developed to admit hundreds of users or more, the mutual relationship should be checked with huge combinations of users. Thus, random scheduling should be used in a communication system adopting the MU-MIMO scheme, and the performance of the communication system is degraded by about 4 to 7 dB compared to the communication system adopting the SIMO scheme.